1. Field of the Invention
The present invention relates to a mixed technology, "smart power", integrated device containing power transistors and control logic and analog driving circuitry combined in a monolithic silicon chip.
2. Description of the Prior Art
The commercial success of so-called "smart power" integrated circuits, wherein the analog signal processing circuitry, the control logic circuitry and the output power devices are conveniently monolithically integrated in a single chip, originated and sprung from the overcoming of compatibility problems among the different fabrication processes relative to the different integrated devices, often necessarily operating under different supply voltages. Most often, even though nonexclusively, the power section of such integrated circuits employs VDMOS transistors which typically require a driving gate voltage level comprised between about 10 V-20 V.
This makes difficult interfacing the power transistor (e.g. VDMOS) with the driving circuitry. Commonly, in fact, the maximum operating voltage of CMOS transistors used in logic circuitry is 5 V. Conversely, the driving circuitry for power transistors operate at about 12 V, while in order to ensure a driving level on the gate of a VDMOS power transistor of 10 V and if a reasonable process "spread" is accounted for, the supply voltage of CMOS transistors of the relative driving circuit should be at least 15 V. Moreover better operating conditions of a VDMOS power transistor (i.e. a lower resistance R.sub.on) may be achieved if the driving voltage level on the gate may be raised to 15-20 V, as it is well known to a skilled technician.
In "smart power" type integrated circuits, this particular problem is commonly obviated by using a "level shifter" circuit, which necessarily makes the driving circuit more complicated.
There is a definite need for the availability of CMOS transistors having an operating voltage of about 20 V in a monolithically integrated semiconductor device of the "smart power" type in order to drive directly the output power transistors with a relatively high voltage without employing special level shifter circuits.
On the other hand, in these integrated circuits there are other CMOS and bipolar transistors which are employed, because of their peculiar characteristics, in control logic circuitry and in signal processing circuitry, respectively. Also the utility and reliability of these other integrated devices could be advantageously enhanced if also these other integrated devices could be made capable of withstanding a higher supply voltage than the voltage normally allowed by the physical structure of these integrated devices produced through a mixed-technology fabrication process.